Tunnel – FHWA InfoTechnology

Tunnel - Laser Shearography (LS)

Target of Investigation

Laser shearography reveals surface or subsurface discontinuities or anomalies, such as delamination, debonding, cracks, and voids.

Description

Laser shearography uses optical measurement for nondestructive testing and quality control applications. It is a noncontact method used to quickly detect defects.

Laser shearography is a full-field interferometry technique used to investigate a test object in a neutral and a loaded state. Out-of-plane deformations between the two states are identified by image processing; these deformations reveal surface or subsurface discontinuities or anomalies.⁽¹⁾

Physical Principle

The physical principle of shearography is illustrated in figure 1. The test sample is illuminated using a laser and imaged on a charged coupled device (CCD) camera via a special optical shearing device. The shearing device coherently combines identical but laterally displaced images of the surface of the sample in the image plane. The resulting interferogram of the object wave is then recorded by a CCD camera.

This illustration is a schematic of the shearography system. On the left is a flow chart showing the steps for data processing in a series of blocks and arrows. From the top, the blocks are labeled, “Shearography Image Calibration Device/Data” and “Computer and Imaging Software.” Below these are a third layer of blocks labeled, from left to right, “Monitor Images,” “Test Part Stress Controller and Sensors Vacuum Thermal Vibration,” and “Phase Stepper Controller.” An image with three pairs of dots is shown in the block “Monitor Images.” The rest of the schematic depicts the shearography system’s test setup. There is a variable diffusion beam laser, a charged coupled device camera, phase stepper, phase shift mirror, beam splitter, two axis tile mirror, and a test part stress device. The devices in the center are focusing fan-shaped beams on a vertical rectangle on the right labeled “Test Part.” Two lines labeled “P1” and “P2” hit and reflect back from the surface of the test part at different locations. — Figure 1. Illustration. Schematic diagram of the shearography system.(2)

Local deformation can be induced by any combination of thermal, acoustical, or mechanical loading under static or dynamic situations. The loading situation results in a different interference fringe pattern that is directly related to the deformation state. Local surface deformations caused by defects inside the object can be found by comparing the interference fringes before and after loading. Only specimens with surface roughness of one wavelength of light or more produce accurate results.⁽⁵⁾ Smooth surfaces will produce random interference and incorrect inspection results, which can usually be addressed by using talc or fine dust on the surface.

Data Acquisition

Users of laser shearography equipment should refer to the manufacturer’s manual. ASTM E2581-14, Standard Practice for Shearography of Polymer Matrix Composites and Sandwich Core Materials in Aerospace Applications, provides general information.⁽³⁾ Raw data can be kept as a permanent record for future processing and evaluation. With some software, the data can be stored in JPEG, TIF, or other image formats.

Data Processing

Usually, the shearography system automatically processes data it collects to attain higher resolution, lower noise, and clearer presentation. Extra data processing can be performed with the software provided by manufacturers.

Data Interpretation

Flaw areas are indicated in phase maps; a butterfly fringe pattern shows the first derivative of the out-of-plane deformation (figure 2-A).⁽⁴⁾ An unwrapped phase map is a plot of the test object’s surface-deformation derivative without fringes (figure 2-B).

This image shows two adjacent centers with alternate white and black stripes surrounding them. — Figure 2. Images. Phase maps from laser shearography show the first derivative of surface deformation.(3)

This image shows two half circles located in the center of the figure. The half circle on the left is black and the half circle on the right is white. The colors transition from pure black or white to grey near the boundaries of the half circles.

Advantages

The laser shearography method has the following advantages:⁽¹⁾

High sensitivity. Can measure surface displacements in the submicrometer range.
Full field coverage. Can measure, record, and interpret data over a large area at one instance.
Noncontact. Does not require contact with the specimen, so it has potential as a high‑speed method.
Real time. Displays near-real-time phase images with live and active results.

Limitations

The laser shearography method has the following limitations:

Only applicable for specimens with surface roughness of one wavelength of light or more.⁽⁵⁾
An experienced individual is needed to interpret the results of the shearography test.
Adequate overhead lighting of the specimen is required to achieve best results.
Thermal deformation of an object may be detected, and the effects of such deformation must be considered in data analysis.

References

Dantec Dynamics. (2018). “Non-Destructive Testing (NDT) with Laser Shearography.” (website) Skovlunde, Denmark. Available online: https://www.dantecdynamics.com/shearography-non-destructive-testing, last accessed January 15, 2019.
Nichols, C. (2013). Nondestructive testing at NASA WSTF, Presentation to the American Institute of Aeronautics and Astronautics, Albuquerque, NM. Available online: https://www.slideshare.net/CharlesNichols1/albuquerque-aia-ac, last accessed January 15, 2019.
ASTM E2581-14. (2014). “Standard Practice for Shearography of Polymer Matrix Composites and Sandwich Core Materials in Aerospace Applications.” Book of Standards 03.04, ASTM International, West Conshohocken, PA.
Yang, L.X. and Hung, Y.Y. (2004). Digital Shearography for Nondestructive Evaluation and Application in Automotive and Aerospace Industries, Proceedings of the 16th World Conference on Non-Destructive Testing, Montreal, Canada.
Asheesh. “Limitations of Shearography Non-Destructive Testing of Composites.” (website) Bright Hub Engineering. Available online: http://www.brighthubengineering.com/
manufacturing-technology/118355-shearography-non-destructive-testing-ndt/, last accessed January 15, 2019.